1. Field of the Invention
The present invention relates to a liquid crystal display element. More particularly, the present invention relates to a liquid crystal display element which can realize a fast response, a broadening of the viewing angle, and an increase of the contrast ratio in a high information content liquid crystal display element to be driven by refresh driving. Furthermore, the present invention relates to a method for driving this liquid crystal display element.
2. Description of the Related Art
The use of liquid crystal elements in various fields such as table computers, watches, hand-held computers, word processors, portable television sets and the like is wide-spread. In these circumstances, the liquid crystal display element is now required to function as a display device, replacing the cathode ray tube (CRT). CRT is an emissive display that can cause asthenopia in an operator, and since the CRT depth is great, the size of the entire apparatus is increased and the design of the apparatus is restricted. Accordingly, the development of a compact flat panel reflection type display which does not cause eye fatigue is strongly desired. Some of the liquid crystal displays used as such elements almost meet this requirement, but still fail to match the CRT from the viewpoint of functions. Especially, the display speed is as low as several hundreds milliseconds per picture, and these liquid crystal displays are not practical for an animation display. Although a quasi animation display becomes possible if the rewriting frequency per unit time is reduced, disadvantages such as a narrowing of the viewing angle and a reduction of the contrast ratio arise. Therefore, the development of a liquid crystal element having a fast response time, a wide viewing angle, and a high contrast ratio is strongly desired.
Conventional refresh drive liquid crystal displays are divided into a twisted nematic (TN) type and a supertwisted birefringence effect (SBE) type, which is a modification of the TN type. In each display mode, as shown in FIG. 4, liquid crystal molecules are continuously twisted from top to bottom by 90.degree. to 270.degree. by an orientation treatment of aligning the liquid crystal and applying the aligned liquid crystal on upper and lower substrates. The resulting panel is placed between two polarizing films and a display is realized by changing the alignment of liquid crystal molecules by switching an electric field ON and OFF.
The TN and SBE used in word processors and hand-held computers are of the refresh drive type and a high information content display is possible to a certain extent. Accordingly, TN and SBE can be used as character displays. In the TN and SBE type liquid crystal displays, liquid crystal molecules aligned horizontally to the substrates, say homogeneous alignment are twisted toward the lower substrate from the upper substrate, as shown in FIG. 4, and the liquid crystal layer as a whole has a birefringence effect on the light incident in the direction vertical to the substrates, to effect switching in the oscillation direction of the light. Thus, in conventional TN and SBE displays, since the display is effected by rotation of the oscillation plane of the light by a continuous change of the direction of the group of liquid crystal molecules, if an electric field drive is applied, the direction of individual liquid crystals is changed from a horizontal alignment to a vertical alignment, against the mutual action among the liquid crystal molecules, and scores of milliseconds to several hundred milliseconds are necessary for this change of the direction. Especially, in an SBE capable of a high information content display, the ratio of the effective voltage Vs applied to a selected point in the time division drive by a matrix electrode to the effective voltage Vns applied to a non-selected point, that is, the ratio Vs/Vns, can be reduced. Namely, since the rise of the light transmission is equal to the applied voltage, the value of the voltage applied when switching between selected and non-selected points is small, and thus the force for changing the direction of liquid crystal molecules is weak and the response is delayed. This is because, since the alignment direction of liquid crystal molecules is continuously changed from the upper substrate to the lower substrate, the force for orientating the liquid crystal molecules is given a continuous gradient along the thickness direction of the liquid crystal layer, with the result that the direction is gradually changed from portions where the orientating force is weak. In an SBE capable of a high information content display with a relatively high contrast ratio, although the response is delayed, the display background is tinged with yellow or blue, and therefore, the SBE is defective in that a color display by a color filter system is practically impossible.
Under this background, as a means for solving the foregoing problems, there has been proposed a display element utilizing a ferroelectric liquid crystal. More specifically, a ferroelectric liquid crystal display (FLC) reported by Noel A. Clark et al [Appl. Phys. Lett., 36, 899 (1980)] is characterized in that a liquid crystal of the chiral smectic C or chiral smectic H phase is aligned in parallel to the substrate. If the liquid crystal of the chiral smectic C or chiral smectic H phase is uniformly aligned in parallel to the substrate, the directions of dipole moments of individual liquid crystal molecules become uniformly upward or downward relative to the substrate, with the result that a spontaneous polarization is caused. By reversing the direction of this spontaneous polarization upside down to the substrate, light switching is effected. This method is the FLC system. In the FLC system, since the spontaneous polarization of the liquid crystal is utilized as the driving source, a fast response is made possible by a strong coupling of the spontaneous polarization with an external electric field, and FLC a memory function such that the once reversed spontaneous polarization maintains this state unless an electric field having a reverse polarity is applied again.
However, since the FLC is fabricated by using a chiral smectic C or chiral smectic H liquid crystal having a layer structure, it is difficult to align individual molecules in parallel to the substrate without destroying the layer structure, and even if a desired alignment is obtained, since the layer structure is thermally unstable, the layer structure is easily broken by a heat cycle or mechanical shock. This is a defect of the FLC.